1. Field of the Invention
The invention relates to a continuously variable transmission mechanism incorporating a variator having a first sheave of adjustable effective diameter on a first axis, a second sheave of adjustable effective diameter on a second axis and a flexible drive band interconnecting the two sheaves, each sheave comprising an axially fixed pulley half, an axially moveable pulley half, a V-groove for the drive band formed by inclined mutually facing surfaces of the pulley halves and adjustment means for urging the moveable pulley half towards the fixed pulley half to adjust the width of the V-groove and thus the effective diameter of the sheave for engagement with the band.
2. Description of the Prior Art
With such a variator it is normally desirable to maintain the drive band in alignment with the two sheaves in the interests of the efficiency of the variator. With the usual symmetrical V-grooves, and the usual arrangement whereby the effective diameter of one sheave is increased as the other is decreased, it is necessary for the moveable pulley half of the first sheave to lie on the same side of the band as the fixed pulley half of the other sheave so that the moveable pulley half of each sheave faces the fixed pulley half of the other sheave. Then, during adjustment, the pulley half of the sheave which is increasing in effective diameter is moved in the same axial direction as the moveable pulley half of the sheave which is reducing in effective diameter so that the band moves in the same axial direction at both sheaves and alignment is maintained.
A problem of this kind of arrangement is that it normally requires a bulky adjustment means for each sheave adjacent to each moveable pulley half and as the moveable pulley halves are on opposite sides of the band, the adjustment means also lie on opposite sides of the band. Thus the overall axial length of the variator is equivalent to the axial length of the two pulley halves plus twice the axial length of an adjustment means. The overall size of an installation is often critical and it would be desirable to be able to reduce the axial length of the variator. The positioning of the two adjustment means on opposite sides of the variator also tends to lead to difficulty in obtaining access to one of the adjustment means, both for servicing and for providing controls to the adjustment means.
One attempt at solving this problem is known from U.K. Patent Specification No. 1,325,634. According to this prior art, the normally desirable V-groove is replaced by a half V-groove in which the fixed pulley halves have radial rather than inclined faces. The drive band then remains in axial alignment with these faces so that the problem of alignment is solved. However, this proposed solution to the problem introduces several further drawbacks, namely: assymmetric loading on the band tending to distort it and cause it to wear unevenly; an absence of clearance between the outer periphery of the radial pulley half and the drive band; and difficulties associated with obtaining a wide spread of transmission ratios between the two sheaves.